The present invention relates to refrigeration chillers. More particularly, the present invention relates to air-cooled refrigeration chillers and, in particular, chiller the evaporators of which are located remote from the remainder of the chiller components.
Refrigeration chillers operate to cool a liquid, such as water, which is most often used to comfort condition a building or in an industrial process. Generally speaking, refrigeration chillers fall into the category of xe2x80x9cair-cooledxe2x80x9d or xe2x80x9cwater-cooledxe2x80x9d. The terms air-cooled and water-cooled refer to the medium to which hot refrigerant gas in the chiller""s condenser rejects its heat in the course of chiller operation.
In the case of an air-cooled chiller, the chiller is typically located outdoors to enable the hot refrigerant flowing through the system condenser to reject heat to the atmosphere. Most air-cooled chillers are packaged such that all components of the chiller are located outdoors including the system""s compressor, condenser and evaporator.
Historically, evaporators employed in air-cooled chillers, have more often than not been of the shell and tube, direct expansion (DX) type. Relatively cold refrigerant, primarily in the liquid form, is directed into the interior of the tubes that form a DX evaporator""s tube bundle while the liquid medium to be cooled, most typically water, contacts the exterior of such tubes. As refrigerant travels the length of the tube bundle one or more times within a DX evaporator, it absorbs heat from the surrounding medium, and, as a result, is heated, vaporizes and is drawn therefrom by the system compressor.
As is the case in most chillers, a relatively small amount of the lubricant used by the system compressor, such as for bearing lubrication, cooling or sealing purposes, becomes entrained in the compressed refrigerant gas that is discharged from the compressor. The portion of such lubricant that is unable to be separated from the flow stream of gas discharged the compressor remains entrained in the gas stream and makes its way therewith to the system condenser. Such lubricant mixes with the liquid refrigerant that is created by the heat exchange process that occurs in the condenser, and then flows with the condensed refrigerant, through the system""s expansion device and into the system evaporator. In the case of a DX evaporator, because the flow of refrigerant through the evaporator is interior of the tubes in the evaporator""s tube bundle, the lubricant that makes its way into those tubes is capable of being drawn thereoutof and returned to the system compressor by the expedient of maintaining a predetermined velocity in the refrigerant gas flow stream that is drawn out of the evaporator tubes by the compressor.
In some air-cooled chiller installations, the physical location of the installation, the particular application in which it is used and/or the varying nature of ambient weather conditions in the locale in which the chiller is used may require or suggest that the chiller evaporator be located indoors or in a protective enclosure, remote from the remainder of the chiller. The purpose of such remote location is typically to ensure that the evaporator does not freeze. Even when DX evaporators are located remote from the remainder of an air-cooled chiller system, refrigerant velocity is capable of being maintained at a sufficient level in the suction pipe leading from the evaporator back to the system compressor to ensure that lubricant that has made its way to the evaporator is returned to the compressor.
Recently, more efficient and sophisticated evaporators have been designed and have come to be employed in chillers, including those of the falling film type. Falling film evaporators and hybrids thereof do not operate on the direct expansion principle and, instead, are of a type in which the medium to be cooled flows internal of the tubes of the evaporator""s tube bundle while the system refrigerant flows exterior thereof. Liquid refrigerant is distributed, in a falling film evaporator, across the top of the evaporator""s tube bundle in low-energy form and trickles downward therethrough, for the most part vaporizing in the process.
Such heat exchangers are more efficient, with respect to heat transfer, and enable the chiller to function with a reduced refrigerant charge. However, because the refrigerant in such evaporators and any lubricant flowing therewith is disposed exterior of and falls downwardly through the tubes that comprise the evaporator""s tube bundle and because the suction gas drawn out of such an evaporator by the system compressor is typically drawn out of the evaporator shell above the refrigerant distributor, suction gas, as it flows out of the interior of a falling film evaporator, is generally incapable of drawing lubricant out of the evaporator for return to the system compressor. Instead, the lubricant collects at the bottom of the evaporator shell, together with any liquid refrigerant that happens not to be vaporized in its downward travel through the tube bundle.
This circumstance makes the return of lubricant from a falling film evaporator problematic, whether or not the evaporator is located remote from the compressor, and/or may require the use of oil return systems that are complicated and/or expensive to manufacture and/or control. See for instance U.S. Pat. No. 5,761,914, assigned to the assignee of the present invention and incorporated herein, in that regard. The difficulty and expense in returning lubricant to the system compressor from a falling film evaporator is, however, clearly exacerbated when the evaporator is located remote from the remainder of the chiller system.
The need therefore exists for a relatively simple, inexpensive and reliable arrangement by which to ensure the return of lubricant from a falling film evaporator to the system compressor in a refrigeration chiller particularly where such evaporator is located remote from the other components of the chiller system.
It is an object of the present invention to provide for the return of lubricant to the compressor in a refrigeration chiller that makes its way from the compressor to the system evaporator.
More particularly, it is an object of the present invention to return lubricant that makes its way from the compressor of an air-cooled refrigeration chiller to the chiller""s evaporator in the circumstance that the evaporator is located remote from the compressor and, as installed, may be at a height which is physically above or below the compressor.
It is a further object of the present invention to provide for the return of lubricant, in an air-cooled chiller system which employs a remote evaporator of the type in which refrigerant flow is exterior of the tubes that comprise the evaporator""s tube bundle, from the evaporator back to the chiller""s compressor in a manner which is relatively simple, inexpensive, reliable and need not be proactively controlled.
It is a still further object of the present invention to accomplish lubricant return in an air-cooled refrigeration chiller that employs a remote evaporator of the falling film type by enabling the remote evaporator to function, for purposes of lubricant return, generally in the same manner as a DX evaporator.
These and other objects of the present invention, which will be appreciated when the following Description of the Preferred Embodiment and attached Drawing Figures are considered, are accomplished by routing the suction pipe that communicates between the evaporator and the compressor in a chiller system from the upper portion of the evaporator shell, where gas is drawn out of the evaporator, to a location physically below the lubricant-rich liquid pool found at the bottom of the evaporator shell. A lubricant line is disposed so as to be in flow communication with both the lubricant-rich liquid pool at the bottom of the evaporator shell and with the suction pipe, at a location where the suction pipe runs physically below the lubricant-rich pool. Because the lubricant line connects into the liquid pool at the bottom of the evaporator shell and into the compressor suction pipe at a location below the liquid pool, both gravity and head cause the lubricant-rich mixture to flow out of the pool, through the lubricant line and into the suction pipe. Additionally, but not mandatory, by the expedient of appropriately sizing the suction line, mixture flow can be enhanced by the purposeful creation of a pressure differential between the location at which lubricant enters the suction pipe and the interior of the evaporator. The delivery of the lubricant-rich liquid into the suction pipe causes such liquid to become entrained in the refrigerant gas flowing therethrough to the system compressor and oil return is, in turn, accomplished much as if the evaporator employed in the system were a DX evaporator as opposed to one in which refrigerant flow is exterior of the tubes in the evaporator""s tube bundle.